Did Shimano ever make a 6 speed cassette for their freehub (not freewheel)? Or did anybody make a Shimano compatible 6 speed cassette for their freehub?

Birddog

In the early days of Shimano cassette hubs, you built your own six speed cassettes using individual cogs and spacers. The kits came in cool wooden boxes (I still have one).

Yes, The Dura Ace AX and EX series used a six speed freehub and cassette. It also had aero brake levers. it was way ahead of it's time

Yes and no.
Yes, Shimano made a 6 speed freehub in their 600 and Dura Ace lines. The inside 5 cogs were interchangable, but the thread-on outer cog was different for the 600 and DA hubs. At the same time, they also still made a freewheel type DA hub, for old fashioned guys like me.
As for your second question, The freehub idea was so new and advanced, that I don't believe that Shimano ever licenced anyone to make compatable cogs for quite a few years.
(maybe late 90's)

I still have a Shimano 600 6 speed cassette from the early '80s. When a cog wore out I could go to the bike shop and get an individual cog. The cassette does not have a lock ring, the smallest cog just screwed on the cassette body and was tightened by pedaling.

On a bike tour from Germany to Italy I brought a spare 26 cog with me and much to the amazement of my freehub riding friends I changed out one of the smaller cogs for the 26 as we reached the Alps. The 42/26 gearing was considered pretty whimpy back then, but I thought it was revolutionary.